Video signals may be characterized by multiple parameters, such as bit-depth, color space, color gamut, and resolution. An important aspect of a video signal's characteristic is it dynamic range. Dynamic range (DR) is a range of intensity (e.g., luminance, luma) in an image, e.g., from darkest darks to brightest brights. As used herein, the term ‘dynamic range’ (DR) may relate to a capability of the human psychovisual system (HVS) to perceive a range of intensity (e.g., luminance, luma) in an image, e.g., from darkest darks to brightest brights. In this sense, DR relates to a ‘scene-referred’ intensity. DR may also relate to the ability of a display device to adequately or approximately render an intensity range of a particular breadth. In this sense, DR relates to a ‘display-referred’ intensity. Unless a particular sense is explicitly specified to have particular significance at any point in the description herein, it should be inferred that the term may be used in either sense, e.g. interchangeably.
A reference electro-optical transfer function (EOTF) for a given display characterizes the relationship between color values (e.g., luminance) of an input video signal to output screen color values (e.g., screen luminance) produced by the display. For example, ITU Rec. ITU-R BT. 1886, “Reference electro-optical transfer function for flat panel displays used in HDTV studio production,” (March 2011), which is included herein by reference in its entity, defines the reference EOTF for flat panel displays based on measured characteristics of the Cathode Ray Tube (CRT). The EOTF for BT. 1886 is characterized by an exponential power function of the form:L=a(max[V+b),0])γ,  (1)where V denotes a normalized video input signal, L denotes screen luminance in cd/m2, and a, b, and gamma (γ) are variables. For example, historical video systems use a gamma of 2.4. As used herein, the term “gamma-coded signal” may denote a video signal coded using a traditional gamma-based EOTF, such as BT. 1886. Most signals of standard dynamic range (SDR) are indeed gamma-coded signals.
Most commercially available professional monitors are characterized by relatively low dynamic range (e.g., 100 to 500 nits); however, newer professional monitors, such as those demonstrated by Dolby Laboratories, may allow for displaying signals at much higher dynamic rates (e.g., 1,000 nits to 5,000 nits, or more). Such displays may be defined using alternative EOTFs that support high luminance capability (e.g., 0 to 10,000 nits). EOTFs are traditionally optimized according to the characteristics of the human visual system (HVS). Signals are coded and quantized based on how many “just noticeable difference” (JND) steps are perceived by human vision. While a gamma-based function imposes a fair degree of perceptual uniformity for SDR signals, HDR signals require perceptual uniformity over a much wider range. An example of such an EOTF is defined in SMPTE ST 2084:2014 “High Dynamic Range EOTF of Mastering Reference Displays,” which is incorporated herein by reference in its entirety. Another example of a perceptually-quantized EOTF is presented in “Chromaticity based color signals for wide color gamut and high dynamic range,” by J. Stessen et al., ISO/IEC JTC1/SC29/WG11 MPEG2014/M35065, October 2014, which is incorporated herein by reference in its entirety. As used herein, the term “PQ-coded signal” may denote a video signal coded with a perceptual quantization function, such as the ones described in SMPTE ST 2084 and the M35065 submission. Compared to the traditional gamma curve, which represents the response curve of a physical cathode ray tube (CRT) device and coincidently may have a very rough similarity to the way the human visual system responds, a PQ curve imitates the true visual response of the human visual system using a relatively simple functional model. The inverse of the EOTF, (commonly referred to as OETF or EOTF−1) is typically applied to the signal by the transmitter. Given a video stream, information about its EOTF or OETF is typically embedded in the bit stream as metadata and there is a direct correspondence between an EOTF and its inverse; thus, given an EOTF, one can determine its OETF, and vice versa.
In recent years, there has been an increased interest in compressing video signals. Examples of video compression methods include both standards-based techniques (such as MPEG-2, MPEG-4, H.264, H.265, and the like) and proprietary techniques (such as Windows Media Video from Microsoft, and VP8 and VP9 from Google). Most of these compression techniques target the efficient coding of gamma-coded SDR signals (typically, up to a bit depth of 10 bits). As appreciated by the inventors here, with the proliferation high-dynamic range images, improved compression techniques for PQ-coded images are desirable.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.